The present invention relates generally to the field of radio communication, and more specifically, to the field of pilot signal search window control in code division multiple access (CDMA) cellular telephones.
The primary standard specification relevant to the present invention is TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System. This industry standard specification is considered understood by those reasonably skilled in the art of the present invention. Specific sections of relevance include all of section 6.6, with particular emphasis on sections 6.6.6.1-6.6.6.2.3, and section 7.7.2.3.2.1.
After a mobile station acquires a pilot signal from a base station, the base station continually instructs the mobile station to search at specific locations in time for pilot channel CDMA energy from both the present base station and other base stations. Other energy from the present base station is important since CDMA systems are able to diversity combine multipath reflections of the same signal into usable energy through the use of multiple digital receivers, often referred to as demodulating "fingers". In addition, information about the time location of other base stations is important from a cellular soft-handoff standpoint. Besides providing the specific locations to search for pilot signal energy, the base station also provides the mobile station a search window size for each pilot signal. The available search window sizes range from 4 chips up to452 chips. Thus, the time range around each specific time location is defined by the base station as a search window size.
One physical implementation of CDMA searching mechanisms includes using a mobile station modem application specific integrated circuit (MSM ASIC) and a central processing unit (CPU) with access to memory and a direct memory access controller (DMA controller). Such an MSM ASIC includes a searcher receiver and multiple digital receivers, all of which are controlled through control registers accessible by the CPU. As a pilot search is performed, result data is stored directly into the CPU memory through the DMA controller. After the search is complete, an interrupt is generated which causes the CPU to analyze the result data and re-assign the digital receivers as necessary. Additionally, a search for the next pilot signal is also initiated.
Unfortunately, one of the problems associated with such an implementation relates to CPU processing availability. When search windows are small, the CPU is more often required to respond to search completion interrupts and analyze result data. Thus, this process can utilize too much CPU processing time and resources. One way of addressing the problem of having too many search completion interrupts would be to simply overide the recommended search size by requiring that all searches be larger than a predefined amount. However, simply overriding the search window sizes recommended by the base station would defeat the reasons for having small search windows. One of these purposes is to restrict each search to isolating a single pilot signal. Thus, for areas with high densities of base stations, it would be difficult to prevent interference between the base stations without the benefit of smaller search windows. Another way of addressing the problem would be to introduce a delay between searches. Unfortunately, implementation of a timer to accomplish such a delay would also add an extra degree of complexity to CPU operation.
There is, therefore, a need in the industry for a method and an apparatus for addressing these and other related, and unrelated, problems.